Mechanical Motion Transformers and Coupling Elements

Mechanical Motion Transformers

A. Rack and Pinion

This system uses a normal gear meshing with a gear of infinite radius (rack). The rack's teeth are trapezoidal, and the system's operation is reversible. Depending on which element is driving and which is fixed, we have:

• Pinion rotates, rack fixed: The pinion moves linearly. This is used in winches for manual carriage movement.
• Rack moves, pinion fixed: The pinion rotates. This has few current applications, formerly used in mechanical calculators.
• Pinion rotates in place: The rack moves linearly. This is used in drill presses, automobile steering, and automatic garage doors.

B. Screw-Nut

The most common application involves rotating the screw while preventing the nut from rotating, causing the nut to move longitudinally.

Applications:

• Moving loads and holding objects: Car jacks, screw clamps.
• Precise object placement: Adjustment mechanisms in binoculars.

C. Cam and Eccentric

These elements transform circular motion into reciprocating linear or circular motion, and this transformation is not reversible.

Eccentric: A disc or cylinder whose rotation axis is offset from its geometric center. This offset is called eccentricity. The follower's motion is smooth and continuous.

Cam: A shaped metal or plastic piece attached to a shaft, which displaces a follower. There are two types:

• Linear cams: Few applications.
• Cam presses: The follower's maximum displacement is known as the cam's lift. A spring returns the follower.

Classification by follower type:

• Peripheral: Cause reciprocating linear motion, depending on the cam's shape. Contact can be direct or indirect.
• Oscillating: The follower describes a circular arc. Lubrication is important.

Classification by cam shape:

• Plate or Disc: Most common type, used for opening and closing valves in internal combustion engines.
• Cylindrical: Rotation causes linear displacement of a rod.
• Box or Conjugate: Complex to manufacture, used for specific applications.

D. Crank-Connecting Rod-Piston

This mechanism can transform circular motion into linear, and vice-versa.

Circular to Linear: The crank (connected to an engine's axle) drives the piston. The piston's linear motion powers the machine (e.g., reciprocating saw, air compressor).

Linear to Circular: Used in internal combustion engines.

E. Ratchet

Ratchets prevent shaft rotation in one direction while allowing it in the other. They consist of a toothed wheel and a pawl, which engages the teeth. There are reversible and non-reversible types.

F. Freewheel

Allows a driveshaft to rotate a driven shaft in one direction but not the other, decoupling the shafts when the driven shaft tries to drive the driveshaft. Consists of two wheels, rollers, and springs.

Mechanical Coupling Elements

The most important coupling systems are:

• Detachable: Allow easy separation without damage.
• Fixed: Not intended for separation during normal operation; removal requires breaking the joint.

A. Detachable Unions

• Threaded:
  • Bolt and Nut: A screw that threads into a nut. The joined pieces are not threaded.
  • Screw: Similar to a bolt, but the threaded hole acts as the nut.
  • Set Screw: Threaded into one piece and prevents movement of the other.
  • Stud: Threaded at both ends, used to join large parts with smaller, removable parts.
  • Tapping Screw: Creates its own thread in thin metal.
  • Lag Screw: Used for joining wood.
• Other Joints:
  • Pins: Cylindrical or conical pieces for holding elements together.
  • Keys: Prismatic steel pieces that transmit torque between two parts.
  • Splines: Allow longitudinal movement while transmitting torque.
  • Guides: Facilitate linear movement.
  • Button and Buttonhole: Common in textiles.
  • Velcro: Plastic strips that adhere to each other.
  • Hooks: Metal pieces for fastening garments.

B. Fixed Unions

• Rivets: Cylindrical rods with a head, used for permanently joining thin sheets or pieces.
• Roblón (Large Rivet): Rivets with a diameter greater than 10mm.
• Press Fit: A shaft with a larger diameter than the hole is forced into place.
• Bonding: Using an adhesive layer to join surfaces.
• Welding: Joining materials by heat or pressure.